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Problem 5—What are "key de-bouncing" circuits?

When a mechanical switch is turned on or off, the contacts can make and break the circuit dozens or hundreds of times before settling down into one position. These makes and breaks are highly undesirable in digital circuits like counters. For example, if we have a counter to count the number of times a switch is pressed, and if the key is mechanical; then whenever the key is pressed once, the counter advances many times instead of just once.

Some special circuits are used to prevent these extra makes and breaks of the mechanical switches. These circuits are called "key de-bouncing" or "contact de-bouncing" circuits. These circuits provide only one make or break per activation of the switch. A basic "key-debouncing" circuit using S-R flip flops is shown below.